Radio Shack will have speaker wire.They tend to be a little overpriced, so if you can find a general hardware store they might sell wire by the foot and have a lot of sizes to choose from. Then you can get just what you want and probably pay less.
Wire sizes are denoted by gauge, with higher numbers being thinner. 16 gauge is common for speaker wire. 18 to 22 gauge is usually what's included in computer speakers. Heavier wire allows for more wattage without signal degradation. For the 680s I'd go with 16 or 18 gauge. You can usually get it in pairs, like what you got with the speakers, with one wire having a stripe on the coating. That stripe is important.
Once you get all the wire you neeed and cut it into the desired lengths, there are a few ways to strip the ends. First you'll need to split one wire from the other. Use a knife to divide them, then pull them apart as far as you like (usually just a couple of inches). From there you have three options for stripping the ends:
1. Bite down on the coating and pull the wire out of it. This is only recommended if you don't have wire strippers or cutters or a knife or scissors or really anything sharp. And don't tell your dentist you did it.
2. On a pair of wire strippers, find the hole that matches your wire size (there will be numbers next to each hole indicating the gauge) and clamp that hole over the end of the wire. Hold the wire tightly and pull the strippers away, and it should remove the coating from the wire. The problem with this method is that the holes aren't very exact and will sometimes not cut through the coating, leaving a messy strip job. More commonly, they'll cut too far and snip off some of the individual wires inside, leaving you with a pitiful little strand that won't carry much of a current. If you're really careful about cutting it just right, this is probably the easiest method.
3. If you have wire cutters around, close them in on the wire coating (not too snug) and rotate them around the wire a couple of times. They'll cut through the coating like a pipe cutter goes through a pipe, and then you can grap the handle (don't squeeze!) and pull the tool toward the end of the wire, removing the coating. That's how I like to do things.
Take about an inch of coating off each end. Then twist the wires so that they don't fray. As a final step, heat up a soldering iron, apply it to the bare wire, and feed in a bit of solder. That's what is known as "tinning" the wire, and it helps to keep ends from fraying or breaking. You'll notice that Logitech has already done that with their wires.
Is that clear enough? Let us know if you have any trouble.